1. Field of the Invention
This invention relates to a lead frame for an IC which is provided in the inner lead unit thereof with an aluminum coating.
2. Description of the Prior Art
Among the existing packaging systems for semiconductor integrated circuit (IC) elements is counted the ceramic package introduced by U.S. Pat. No. 4,604,291. This package is produced by preparing a ceramic substrate possessing at the center thereof a depression for accommodating a given IC element, fusing a lead frame made of a copper alloy or an iron-nickel alloy with low melting point glass to the ceramic substrate, separately preparing a ceramic lid having similar low melting point glass fused thereon so as to be paired with the ceramic substrate, fixing the IC element in the depression of the substrate, connecting the electrodes on the IC element to the metal coating of an inner lead unit of a lead frame through the medium of fine metallic wires, mounting the lid on the substrate, and heating the substrate and the lid jointly, thereby sealing the gap therebetween in an air tight manner. Since this package has a highly satisfactory property of radiating heat, it finds utility in IC's of relatively high-power and high-speed.
Particularly where the package is required to operate with high reliability and the electrodes on the IC element are made of aluminum (Al), the practice of forming the fine metallic wires with Al and also forming the metallic coating on the wire bonding part of the inner lead unit of the lead frame with Al is generally followed.
One of the various versions of the packaging system described above necessitates an extra step of folding an external lead unit after completion of a package. If the seal glass used in this package has a low melting point, the possibility arises that the glass layer will sustain a crack during the folding of the external lead unit and the package will consequently lose airtightness. To preclude this trouble in the production of an IC device which requires this folding step after completion of packaging, the practice of using seal glass of high strength has been gaining in popularity. Since this glass has a high melting point, the work of fusing the substrate with the lead frame must be carried out at a temperature in the range of 500.degree. to 530.degree. C., a level representing an increase of about 100.degree. C. over the temperature prevalent in the production using the conventional low melting point glass. When the fusion of the lead frame is carried out at this elevated temperature, Cu, Fe, Ni, etc., which are component elements of the lead frame, are caused to diffuse and reach the surface of the lead frame and impair the connection of Al wires and jeopardize the fastness of their joint so long as the Al coating has a thickness in the conventional range of 2 to 2.5.mu.. This drawback is tentatively overcome by increasing the thickness of the Al coating to at least twice the conventional magnitude. This increase in the thickness of the Al coating inevitably results in a proportionate increase in the time required for the work of fusion and in the cost of production as well. Under the circumstances, the desirability of seeking a drastic solution to the problem has been finding growing recognition.
In the light of the true state of affairs described above, the present invention aims to provide a lead frame for an IC which is enabled to give effective prevention of the component elements of the lead frame thereof from being diffused in the Al coating and which can be manufactured at a low cost.